What is Listeria and How Does it Affect Food Safety?

What is Listeria? Listeria monocytogenes (L. monocytogenes) is a bacterium that can cause a serious infection in humans called Listeriosis. Listeria monocytogenes is commonly found in soil and water and can contaminate food, particularly raw or undercooked meat, unpasteurized dairy products, and certain fruits and vegetables.

The worst listeriosis outbreak on record occurred in South Africa in 2017-2018. The outbreak was traced to a ready-to-eat processed meat product called “polony,” and resulted in over 1,000 confirmed cases of Listeriosis and over 200 deaths.

How Does Listeria Monocytogenes Affect Food Safety?

Listeria monocytogenes bacteria can be found in a variety of environments, including soil, water, and animal digestive tracts. Because it is found in animal digestive tracts, it ends up in the animal’s faeces.

Fresh vegetables may be contaminated through soil or the use of manure as fertilizer. Ready-to-eat food can also become contaminated during processing and the bacteria can multiply to dangerous levels during distribution and storage.


In the environment, Listeria can persist in biofilms. These complex communities of microorganisms that attach to surfaces and are often difficult to remove. Biofilms can form on food contact surfaces, such as cutting boards, knives, and other equipment used in food processing.

Listeria monocytogenes are often found in food-handling environments such as processing facilities, kitchens, and refrigeration units. Listeria can survive and even grow at low temperatures and in the presence of salt and other preservatives. This means that it can potentially contaminate a wide range of foods and pose a serious health risk to vulnerable individuals, which makes it a significant food safety hazard and concern for the food industry.

Examples of Food Most Often Associated with Listeriosis

Meat and Cheese Platter
Cold Meat and Certain Cheeses

It’s important to note that not all products within these categories are necessarily associated with listeriosis, and other foods may also be sources of the bacteria.

  • Unpasteurized (raw) milk and dairy products.
  • Ready-to-eat deli meats and hot dogs.
  • Soft cheese made with unpasteurized milk, such as Feta, Brie, queso fresco, and Camembert.
  • Refrigerated meat spreads and pâtés
  • Raw or smoked fish and seafood, including smoked salmon, lox, and sushi.
  • Raw or lightly cooked vegetables, including sprouts.

Who is at Risk of Listeriosis?

L. monocytogenes is especially pathogenic to high-risk human populations such as:

  • newborns
  • pregnant women
  • the elderly
  • people with weakened immune systems, such as persons immunocompromised by anticancer drugs, graft-suppression therapy and AIDS

Other conditions that may increase susceptibility to Listeriosis are:

  • diabetes
  • cirrhosis
  • asthma
  • ulcerative colitis
  • end-stage renal disease
  • liver disease
  • alcoholism

Healthy people are generally at low risk of contracting L. monocytogenes-related illnesses, but heavily contaminated food can make any person susceptible to infection.

Although listeriosis infections are relatively uncommon, it is a potentially fatal disease.  Frequent spontaneous abortions do occur in pregnant women who are infected with the bacteria.

Even though the symptoms may be relatively mild in the mother, the infection may be transferred to the foetus, causing severe illness or foetal death.

The Symptoms of Listeriosis

The disease’s onset can occur anywhere from a few days up to 6 weeks after the ingestion of L. monocytogenes bacteria.  The symptoms last from a few days to several weeks.

Listeriosis is clinically defined when the bacterium is isolated from blood, cerebrospinal fluid, or otherwise sterile site (such as the placenta).

Symptoms of L. monocytogenes may include:

  • fever
  • stiff neck
  • confusion
  • weakness
  • vomiting sometimes preceded by diarrhoea
  • meningitis
  • encephalitis
  • septicaemia
  • spontaneous abortion
  • stillbirth
  • influenza-like symptoms

The Responsibilities of Food Handling Organisations

The South African Listeriosis outbreak has resulted in an unprecedented public health risk.  The industry could have easily avoided this.  Food handling organisations have to ensure that steps are taken at all production levels to prevent contamination of meat products by Listeria monocytogenes.

While this is a difficult task, given that L. monocytogenes is so widespread in the environment, the 2017-2018 outbreak has shown that it is a necessary precaution.

Food-processing operations need to make every effort to prevent L. monocytogenes in raw, unprocessed foods and recontamination in precooked, ready-to-eat finished products.

Due to foodborne illness associated with L. monocytogenes in meat and poultry products, the USDA has issued a zero-tolerance policy for the organism in ready-to-eat foods.

How to Effectively Control Listeria Contamination

Effective control of L. monocytogenes is challenging.  It requires intensive management and extensive resources.

Even though the risk of contracting listeriosis is relatively low, the consequences are devastating for both the consumer and the processor when it does occur.

The presence of L. monocytogenes in raw ingredients emphasises the importance of adequate cooking (thermal processing) to destroy the organism.

Preventing the spread of Listeriosis starts with proper housekeeping and cleaning. Factory employees need to ensure they maintain good hygiene practices all the time. Factories need to be thoroughly cleaned and disinfected. Proper verification systems, including taking micro swabs, visual observation, etc., become necessary to ensure the effectiveness and efficiency of cleaning.

To prevent listeria contamination in food, food companies can take the following steps:

  • Implement Good Manufacturing Practices (GMPs): GMPs are a set of guidelines that help ensure the safety and quality of food products. These practices include maintaining clean facilities, training employees on food safety, and monitoring equipment and utensils for cleanliness.
  • Use Hazard Analysis and Critical Control Points (HACCP) systems: HACCP is a systematic approach to identifying and controlling potential hazards in the food production process. This involves identifying potential hazards, establishing critical control points (CCPs), monitoring the process at those CCPs, and taking corrective action if necessary.
  • Maintain proper hygiene: Food companies should ensure that all surfaces, equipment, and utensils that come into contact with food are properly cleaned and sanitized. This includes regular hand washing and wearing protective clothing, such as gloves and hairnets.
  • Control temperature: Listeria can grow at refrigeration temperatures, so it’s important to ensure that food products are stored and transported at the proper temperatures to prevent bacterial growth.
  • Conduct regular testing: Food companies should regularly test their products and equipment for listeria contamination. This can include swab testing of equipment and testing of finished products before they are shipped.

By following these steps and implementing a comprehensive food safety program, food companies can help prevent listeria contamination in their products and protect the health of consumers.

How Training Can Help Prevent Listeria Contamination

Staff training in a food handling environment also plays a vital role in controlling contamination problems.  Employees must understand the organism, observe basic sanitation principles, and gain the same sense of personal responsibility exhibited by management and regulatory officials.

Here are some ways that training can help prevent Listeria contamination in your food-handling facility:

  • Training in proper food handling techniques: Proper training in food handling techniques can help prevent contamination of food with Listeria. This includes measures such as cleaning and sanitising hands and surfaces that come into contact with food, storing food at the proper temperature, and using separate cutting boards and utensils for raw and cooked foods.
  • Training in good cleaning and sanitation practices: Training in proper cleaning and sanitation techniques can also help prevent the growth and spread of Listeria. This includes cleaning and sanitising surfaces and equipment that come into contact with food, such as countertops, cutting boards, and knives.
  • Training in temperature control: Cooking food to the appropriate temperature can help kill Listeria bacteria. Training in proper cooking techniques and temperature control can help ensure that food is cooked thoroughly.
  • Training in good storage practices: Proper storage of food can help prevent the growth of Listeria. Training in proper storage techniques, such as refrigerating or freezing food promptly, can help reduce the risk of contamination.
  • Training in good personal hygiene practices: Training in proper personal hygiene, such as washing hands regularly and avoiding cross-contamination, can also help prevent the spread of Listeria.

In addition to training, it is important to follow food safety guidelines and regulations to prevent Listeria contamination.

ASC Consultants has a wide range of training modules available to help employees understand the dangers of biological food safety hazards, including Listeria and its preventative measures.



To ensure food safety, handling and storing food properly, especially high-risk foods that are more likely to be contaminated with Listeria is important.

This includes keeping foods at the correct temperature, washing hands and surfaces often, cooking food thoroughly, and avoiding cross-contamination between raw and cooked foods. It is also important to follow food safety guidelines when purchasing and preparing ready-to-eat foods, such as deli meats and soft cheeses, and to consume them before expiration.

Awareness is key!

You are welcome to drop us a comment in the comment section below.  We love to hear from our website visitors!

2 thoughts on “What is Listeria and How Does it Affect Food Safety?”

  1. Listeria outbreaks are very scary. There have been many Listeria outbreaks reported worldwide every year. Most of these outbreaks are mainly due to the lack of proper food handling practices. This is a very informative article about Listeriosis. It is important to know what Listeria is and whom is most susceptible to contracting this potentially deadly disease. I am very careful when handling and preparing food and do not take any chances. If I have any doubts about any food products I don’t buy it or it gets tossed.

    • Hi Monika,
      Thank you very much for having a look at our post. Good food handling practices are not only the manufacturer’s responsibility, but the consumer should also be vigilant when preparing and cooking food.


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